1. Field of the Invention
The invention relates to a power conversion equipment and a control device of the power conversion equipment, and more particularly, to a power conversion equipment and a control device capable of preventing generation of over current at the time of a fault in an AC system.
2. Description of the Related Art
In a conventional control device for a power conversion equipment, generation of over current at the time of a fault in an AC system has been prevented by increasing the response speed of the controller for the power conversion equipment, that is, performing dq coordinate transformation of an output AC current of the conversion equipment and performing current control of a d-axis component and a q-axis component with non-interaction at high speed.
In this method, however, there is no control of a negative-phase-sequence component. Therefore, when AC voltage contains a negative-phase-sequence component due to an unbalanced fault or the like in an system, such negative-phase-sequence component is not reflected on a control output and causes, in some cases, flow of an over current due to the imbalance between voltages of the AC system and the conversion equipment.
Japanese Patent Laid-Open No. Hei 4-4812 discloses a power conversion equipment, in which generation of an over current due to an imbalance fault or the like is prevented by providing a negative-phase-sequence component detection circuit for detecting a negative-phase-sequence component using a band pass filter and controlling the power conversion equipment through detection of a negative-phase-sequence component by the negative-phase-sequence component detection circuit.
With the above-mentioned conventional control device for power conversion equipment, the filter for extracting a negative-phase-sequence component used in a steady state is the same as that used in a transient state on the assumption that any negative-phase-sequence component is not contained in a steady state. Therefore, the filter for extracting a negative-phase-sequence component in the transient state, as in an imbalance fault or the like, that is, the filter intended for the transient state, is also used in the steady state to extract a negative-phase-sequence component for control of the associated power conversion equipment.
Actually, a negative-phase-sequence component contained in a system voltage rapidly increases in a transient state as in an unbalancing system fault or the like. On the other hand, such a negative-phase-sequence component is almost never present but is present, on the order of several percent (%), in the steady state. Thus, negative-phase-sequence components in a steady state and in a transient state differ in generation and properties, so that, in order to extract a negative-phase-sequence component and control the conversion equipment in a suitable manner, appropriate filters must be used in the steady state and in the transient state, respectively.
However, in the above-mentioned conventional control device f or the power conversion equipment, the filter intended for the transient state is used in a steady state to extract a negative-phase-sequence component for control of the power conversion equipment, and so is not suitable for the steady state. Therefore, there is caused a problem that when a negative-phase-sequence component detected by the filter intended for the transient state is used to control the power conversion equipment in a steady state, control is not correctly performed and an instability is generated.
It is an object of the invention to provide a power conversion equipment capable of dealing with an unbalanced fault by changing characteristics of filters for extracting negative-phase-sequence voltage in a steady state and at the time of a system fault, respectively, and to provide a control device of the power conversion equipment.
Accordingly the invention provides a control device of a power conversion equipment comprising a three phase/two phase transformer for transforming three phase AC voltage of the power conversion equipment into two phase voltage, a positive-phase-sequence component extractor for extracting a positive-phase-sequence component from the two phase voltage, a negative-phase-sequence component extractor including first and second negative-phase-sequence component extraction filters having different extracting capabilities, for extracting a negative-phase-sequence component from the two phase voltage with the use of the first negative-phase-sequence component extraction filter in a steady state, and for extracting a negative-phase-sequence component from the two phase voltage with the use of the second negative-phase-sequence component extraction filter in a transient state, and a control signal generator for generating a control signal on the basis of the positive-phase-sequence component extracted by the positive-phase-sequence component extractor and the negative-phase-sequence component extracted by the negative-phase-sequence component extractor.
According to the invention, in the negative-phase-sequence component extractor, the first negative-phase-sequence component extraction filter is used to extract a negative-phase-sequence component from the two phase voltage in a steady state and the second negative-phase-sequence component extraction filter is used to extract a negative-phase-sequence component from the two phase voltage in a transient state, so that an appropriate control system can be provided, which is conformed to a system condition.
Preferably, the second negative-phase-sequence component extraction filter is one having a higher speed of response than that of the first negative-phase-sequence component extraction filter. In this case, it is possible to provide a control system, which is stable in the steady state and has an adequate speed of response at the time of sudden change in voltage, such as a fault in the system.
The three phase/two phase transformer may employ a three phase/two phase transformation reciprocal both phase transformer for transforming three phase AC voltage of the power conversion equipment into a positive-phase-sequence dq-axis component and an negative-phase-sequence dq-axis component, respectively. In this case, a negative-phase-sequence component appears as a direct current, so that it can be easily extracted.
The negative-phase-sequence component extractor may comprise a filter switching unit for performing switchover of the first negative-phase-sequence component extraction filter and the second negative-phase-sequence component extraction filter on the basis of a negative-phase-sequence voltage. In this case, it is possible to provide a control system, which is free of malfunctioning at the time of an unbalanced fault and has a high speed of response when an unbalanced fault occurs in the system.
The negative-phase-sequence component extractor may extract a negative-phase-sequence component in a steady state with the use of both the first negative-phase-sequence component extraction filter and the second negative-phase-sequence component extraction filter. In this case, it is possible to more accurately extract a negative-phase-sequence component in a steady state as compared with the case where only the first negative-phase-sequence component extraction filter is used.